This came up in a saga I am in, and I thought I'd share my reflexions about it with people, since it may be of interest to some troupes.
And yesterday, while writing about it at home, some further thoughts crept up, so I wrote it a big pile of text, and here it is 
For long, the only way for magi to fly was "Wings of the Soaring Wind". But with the advent of ReCo guidelines, some people have used it to create flight spell, which, IMO, not only makes sense, but is supported both by the guidelines (Base 5: move the target slowly in any direction, even if the target is unspported") and, to some extand, by the spells (Lifting the Dangling Puppet, for exemple).
For the record, I see Wings of the Soaring Wind as a clever work around for the corpus-impaired, but some troupes may disagree and see it as the ultimate flight spell.
Not unlike it, some troupes consider that ReCo "transport instantly" guidelines (like in Wizard's Leap) actually move you at very fast speeds, which implies that, if you use them to cross a chasm, they "fly" you over it.
Conversely, ReCo base 4 and 15 allow to move a target "move a target slowly in any direction you please", but since you've got both ReCo 04 stating "straight up or in one direction over surfaces that cannot support it" (this is what Lifting the dangling Puppet does) and ReCo 05 stating the same thing, plus "even if unsupported", it seems to me that these move a target, but that she needs to be supported, and that these guidelines thus can't be used in a true Flight spell.
It thus seems that one must use the Base 05 to fly.
Now, at what speed does he move, and can he move faster?
The first is easy, we've got exemple spells: "As fast as smoke rises". Nonetheless, some troupes may think this is too imprecise.
The later depends on troupes, but it seems sensible that you can add magnitudes using the standard rules. What does it give?
We’ve got 2 ways of doing this : The crunchy one, and the fluffy one.
The crunchy one needs to define what’s “slowly”, or what’s the speed of smoke rising.
I’ve seen it at about 1 feet per second, although this varies greatly. Let’s say 1 pace per turn. This seems slow enough, and is consistent with other ranges given in Ars.
What would magnitudes add? Based both on the standard 1 mag = *10 rule, and on the exemple of ReCo teleport spells (remember, for some troupes, this sometimes implies veeeeery fast flight), we would have this:
Base 05: 1 pace per turn
Base 10: 10 paces per turn
Base 15: 100 paces per turn. If using 1 pace = 1 meter, this means about 37 mph, comparable to Wings of the Soaring Wind
Base 20 would have it at 1000 paces per turn, which surely would require some kind of protection.
ReCo “teleport” spells are one magnitude higher, but allow instant travel.
Now, the fluffy method just gives descriptions. It is used both for actual ReCo effects and in tC&: On p148, you've got a design for a flying carpet, that flies... "as fast as a horse can run" (base 1, +2 very unnatural movement, +1 for speed).
Having always used this, here’s my speed chart, should people like it. I’ll include conversions, for those interested. It happens to be slower than the above, although this wasn’t the goal. Each magnitude takes you up one step on it.
- As fast as smoke rises. Say, 0,37 mph if this is really important for you.
- As fast as a man can walk. This is, say, 4mph
- As fast as a man can run. Say, 14 mph
- As fast as a horse can run. That’s… about 30 mph
- As fast as an eagle can fly. About 80 mph. This shouldn’t be very comfortable without protection, of at all possible.
- Faster (never happened, would really need protection).
Here it is. I’m sure people will have variants, but, well, this is for those who never thought about it and may like one or the other 
But horse couriers in most cultures have had systems of switching out to fresh horses every so often, as you'd ride the horse to death otherwise. Four-legged mammals do not have as efficient of cooling systems as we twoleggers do.